In dental care, there are cases of cutting semi-hard layers of dentin or the like formed on a tooth surface and in a root canal where a dental rotary drive cutting tool such as a peeso reamer or a gates drill is used for the purpose of cutting such layer. Here, the peeso reamer and the gates drill only differ in shape of cutting edge formed at the tip, and have the same basic structure and function of cutting tooth dentin.
The tools disclosed in Patent Document 1 (Japanese Patent 3375771) are well known as the peeso reamer or the gates drill. FIG. 5 is a diagram showing the structure of the peeso reamer disclosed in Patent Document 1, and FIG. 6 is an enlarged sectional view cut along a line I-I of FIG. 5. As shown in FIG. 5, a cutting portion 1 is formed along a predetermined length of a peeso reamer A from one end, and a shank 2 having a predetermined length is formed on the other end. A neck part 3 constitutes the part between the cutting portion 1 and the shank 2.
As shown in FIGS. 5 and 6, multiple (3 in the drawings) cutting edges 1a are formed in the cutting portion 1, each of the cutting edges 1a has a preset angle of torsion, and the external shape of the side surface is straight.
The peeso reamer A is rotated and operated by gripping a rotary drive device chuck such as a hand piece or the like, which is omitted from the drawings. For this purpose, the shank 2 has a straight whirl-stop 2a corresponding to the chuck.
The neck part 3 is formed straight having a smaller diameter than outer diameters of the cutting portion 1 and the shank 2, and respective connecting parts on either end with the cutting portion 1 and the shank 2 are formed in an R shape having a predetermined curvature radius.
The aforementioned structure is also for the gates drill. However, in the case of the gates drill, while the corresponding portion to the cutting portion 1 shown in FIG. 5 also has multiple cutting edges with a preset angle of torsion, the angle of torsion (angle between cutting edge and axial center) is large, and the structure in which the external form of the cutting portion formed in a spherical shape differs from that of the peeso reamer.
The peeso reamer A formed as described above is made of austenitic stainless steel. Austenitic stainless steel is excellent in that it does not rust, but it cannot be hardened by quenching. Therefore, in Patent Document 1, austenitic stainless steel wire rod is subjected to a cold wiredrawing process so that the crystal structure is pulled along the line length and made thinner into a fiber structure, work hardened, and then used. Making it into such a fibrous structure allows setting a predetermined hardness of Hv 500 or greater, for example.
Moreover, by axially pulling the composition into fibers through cold wire-drawing, improvement in bending strength and exhibition of uniform strength without any fluctuation across the entire length is possible. The aforementioned wire rod is cut to a length corresponding to the target peeso reamer A, making materials, and the materials are then machined to make the peeso reamer A.
The material obtained by cold wire-drawing austenitic stainless steel has a concentric hardness distribution. In other words, hardness at the surface is highest, and gradually decreases toward the center. Thus, it is hard at the cutting portion 1 and the shank 2 and softer at the neck part 3.
When cutting a root canal with the peeso reamer A structured as described above being attached to a hand piece, since the neck part 3 is narrower and softer than other parts, it bends easily along the root canal curve and rotates in the bent state. The cutting edges la cut dentin on the inner wall of the root canal according to this rotation to perform a specific treatment. Since the peeso reamer A rotates in a bent state, a single rotation applies a single-lap bending force on the neck part 3. Through repetitive use, it is bent repeatedly for the number of rotations, and thus the hardness gradually increases and bending fatigue is accumulated. Therefore, the peeso reamer A eventually breaks. However, since the neck part 3 is narrow, the breaking place may be limited to the neck part 3 without breaking at the cutting portion 1 or the shank 2.
When the peeso reamer A breaks at a position near the shank 2 of the neck part 3, the broken end of the neck part 3 protrudes from the root canal. Therefore, the peeso reamer A may be extracted from the root canal by pinching and counter-rotating the protruded portion with pliers or the like. However, if it breaks near the cutting portion 1, the broken portion enters the root canal and cannot be pinched.
As a result, a peeso reamer that has a tapered neck part 5 as shown in FIG. 7 is proposed. This neck part 5 is wider on the cutting portion 1 side and narrower on the shank 2 side. The narrowest portion 5a is near the shank 2. According to this structure, stress when the peeso reamer A is rotated converges at the narrow portion 5a, and when bending fatigue accumulates and it breaks off, the reamer may be easily extracted.    Patent Document 1: Japanese Patent 3375771